For next generation of mobile telecommunications systems 3rd Generation Partnership Project (3GPP) is in the process of defining solutions for user equipment (UE) measurement logging function and immediate reporting function called Minimization of the Drive Tests (MDT). The MDT study aims at assessing the feasibility, benefits and complexity of automating the collection of UE measurements to minimize the need of manual drive-tests. The work under the MDT study should define use cases and requirements for minimizing drive-tests in next generation LTE/HSPA networks. Also, based on the defined use cases and requirements, the MDT should focus on study the necessity of defining new UE measurements logging and reporting capabilities for minimizing drive tests and analyze the impact on the UE.
The use cases for the MDT will be given as following.                Radio coverage optimization        Mobility optimization        Network capacity optimization        Parameterization for common channels        Quality of Service verification        
1. Radio coverage optimization; that is, information about radio coverage is essential for network planning, network optimization and Radio Resource Management (RRM) parameter optimization (e.g. idle mode mobility parameter setting, common channel parameterization), as well as backend network management activities, such as network dimensioning, CAPEX/OPEX planning and marketing. Additionally the detection of coverage problems (e.g. coverage holes, pilot pollution, low user throughput, etc.) in specific areas may be performed.
2. Mobility optimization; that is, mobility optimization is an important part of network operation. Information about mobility problems or failures can be used to identify localized lack of coverage or the need to adapt the network parameters setting, (e.g. in order to avoid too early or too late handover and to improve the handover success rate and overall network performance)
3. Network capacity optimization; that is, the operator may need to be able to determine if there is too much/little capacity in certain parts of the network. Such determination may help to determine placement of new cells, to configure common channels and to optimize other capacity related network parameters.
4. Parameterization for common channels; that is, user experience and/or network performance can be degraded by suboptimal configuration of common channels (e.g. random access, paging and broadcast channels). Detecting problems (e.g. on UL or DL common channel coverage) or analyzing the performance (e.g. connection setup delay) for the procedures associated with common channels, may help network parameter setting and configuration change for system performance optimization.
5. Quality of Service verification; that is, one of the objectives of the network performance analysis is the verification of the quality of service (e.g. user throughput). This may also allow detecting critical conditions and determining the need to change the network configuration, parameter settings or capacity extension.
In the following, UE measurement logs for minimizing drive tests will be described. The measurement logs may be taken at the occurrence of predefined “triggers”, e.g. periodic trigger, a failure event. The following UE measurements (or similar functionality) are considered for UE-internal logging:
1. Periodical downlink pilot signal strength/quality measurements of serving cell and neighbour cells on same and other radio access technologies.
2. Serving cell becomes worse than threshold; that is, radio environment measurements are logged when the serving cell metric becomes worse than the configured threshold.
3. Transmit power headroom becomes less than threshold; that is, transmit power headroom and radio environment measurements are logged when UE transmit power headroom becomes less than the configured threshold.
4. Random access failure; that is, details on the random access and radio environment measurements are logged when a random access failure occurs.
5. Paging channel failure; that is, details of the radio environment, location, time and cell identity are logged at the point when the UE fails to decode the PCCH on the Paging channel for two consecutive times.
6. Broadcast channel failure; that is, details of the radio environment, location, time, cell identity and frequency are logged at the point when the UE fails to read the relevant DL common channels to acquire required system information for camping on a cell.
7. Radio link failure report; that is, radio measurements available at the UE are reported at the RLF occurrence.
The network can request the UE to perform logging of measurements. The UE executes measurements (e.g. periodical downlink pilot measurements) and logs these measurements internally in a sequential manner. Typically, the log stored internally in the UE will contain e.g. some hour of logged measurement information. For post-processing purpose, these logged measurements/events may be tagged with time information (e.g., time stamps).
When the UE has logged measurements the UE indicates to the network that it has an available measurement log. The network may then request the UE to deliver the measurement log. This procedure is illustrated in FIG. 1. The UE sends 10 an indication to a network node, e.g. an eNodeB or a RNC (Radio Network Controller), that it has an available measurement log. The network node then determines 11 whether is wants to request the measurement log. If it determines to request the measurement log it sends 12 a request to the UE. Thereupon the UE delivers 13 the measurement log to the network node.
The network collects measurement logs from several UEs in different cells. By collecting and retrieving information from the measurement logs the network is able to perform optimization of radio coverage, mobility, network capacity and to perform parameterization for common channels and verification of QoS.
It has been agreed upon to include a time stamping for the MDT measurements. However, the timestamp does not need to be very accurate. It is recognized that a simple mechanism is sufficient. For example, during a log activity of e.g. one hour, one can assume that a drift of approximately ±10 s is acceptable.
Solutions using Global Positioning System (GPS), as well as other network-broadcast time-of-day information have been discussed. A mechanism that e.g. uses System Frame Number (SFN) of the current cell as timing source has the potential of providing a very accurate time reference. However, a UE will change cells and occasionally even lose cell coverage causing problems with the time stamping during the performance of UE measurement logging function.
Most user equipments are equipped with a clock. However, in many cases the clocks of the user equipments are not synchronized with a common time base, e.g. the local time. If the UE utilizes the internal clock when time stamping measurement data, problems will arise when the network retrieves time stamped measurement information from the measurement logs and compare the time stamp measurement information from several UEs in a cell.
The document “Study on Minimization of drive-tests in Nest Generation Networks; (Release 9)”, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; 3GPP TR 36.805, V9.0.0, 21 Dec. 2009, pages 1-24, discusses automating the collection of UE measurements, i.e. MDT, by executing measurements in the UE. It further discloses storing the measurements in a measurement log wherein the measurements are linked to a time stamp that is available in the UE and receiving a request to deliver the log in the UE. Thereupon the log is delivered to the network node. However, the accuracy of the time information is underdetermined.
The document “Time stamp achievement and reporting in MDT”, TD TECH, 3GPP DRATFT; R2-102495, 4 Apr. 2010 relates to the issue of the accuracy of time information in MDT. It discusses a mechanism of reporting time stamp via RRC signalling. A relative time is used for the time stamp of the measurement in the UE. The relative time is linked to a counter which is turned on in the UE when measurement is triggered. However, the network node does not know the accuracy of the time information in the log.
The document “Logged MDT principles”, NOKIA SIEMENS NETWORKS et al, 3GPP DRAFT; R2-103191, 4 May 2010 relates to configuration and reporting of a measurement log for MDT but does not mention the accuracy of the time information.